A dual frequency conversion for lowering the received radio-frequency signals to a baseband frequency is generally necessary. This avoids certain problems of disturbance in the case of a single frequency conversion when the radio-frequency signals are received or transmitted.
US Patent Application No. 2005/0017887 discloses just such a radio-frequency signal reception and transmission device, able to perform a dual frequency conversion to lower the frequency of the received radio-frequency signals with a single antenna to a baseband frequency. A dual frequency conversion is also performed to generate radio-frequency signals to be transmitted by the same antenna on the basis of baseband signals.
With a single frequency conversion, the high frequency signals generated by the oscillator stage and supplied to the mixer unit must have a substantially equal frequency to the frequency of the radio-frequency signals. Disturbance signals at harmonic frequencies to the frequency of the radio-frequency signals may be produced. Since the isolation of the mixer is limited, reverse disturbance radiation of the high frequency signals may occur in the direction of the antenna in the radio-frequency signal reception mode. This reverse radiation of equal frequency to the radio-frequency signal frequency is thus not filtered by the input bandpass filter. Moreover, in transmission mode, the radio-frequency signals produced at the equivalent frequency to the high frequency signals from the oscillator stage can influence the oscillator stage. Because of the radiation from the transmitted radio-frequency signals, a voltage controlled oscillator of an oscillator stage is liable to generate high frequency signals of higher frequency than the desired value.
During frequency conversion in a radio-frequency signal reception/transmission device, a problem of noise has been observed, linked, on the one hand, to the amplifier and mixer unit, and on the other hand to the image frequency during frequency conversion. This image frequency is liable to disturb the useful data signals to be removed from the radio-frequency signals picked up, particularly during conversion from a high frequency to a low frequency. The image frequency signal gain at the output of the mixer unit is generally greater than the useful data signal gain, which may complicate the processing of the information contained in the radio-frequency signals picked up.
Several embodiments of radio-frequency signal reception and/or transmission devices have already been proposed in the past for reducing part of the noise, in particular during frequency conversion of the radio-frequency signals picked up. In this regard, one could cite WO Patent Application No 03/009483 which discloses a device of this type for receiving and transmitting radio-frequency signals whose frequency may be equal to 2 GHz and 5 GHz using two filter antennas. During the dual frequency conversion operations, a specific architecture is achieved for one of the mixer units so as to remove the image frequency of the converted signals. A voltage controlled oscillator is arranged for generating first high frequency signals whose frequency is comprised between 2 and 5 GHz such that after the first frequency conversion, the intermediate frequency signal is close to 1.5 GHz.
This solution proposed in WO 03/009483 considerably complicates the structure of this type of radio-frequency signal reception and/or transmission device for removing or attenuating the image frequency disturbance signals. The frequency of the first high frequency signals is only determined to be within the two radio-frequency signal frequencies to be picked up or transmitted. This type of radio-frequency signal reception and transmission device consumes a great deal of electric energy. Consequently, it cannot be fitted to a small portable instrument powered by a battery or electrical energy accumulator.
Like the previous document, JP Patent No 05-244032 discloses a radio-frequency signal communication device with a dual frequency band. For receiving radio-frequency signals, the device includes a first bandpass filter (810 to 826 MHz) and a second bandpass filter (1501 to 1513 MHz), which are connected to the same reception antenna in order to filter radio-frequency signals whose frequency is within the frequency band of the first filter or within the frequency band of the second filter. A first frequency conversion of the received and filtered radio-frequency signals is performed by a first mixer unit using first high frequency signals whose frequency is close to a mean value between the frequencies of the signals filtered by the first or second bandpass filters. In this manner, the frequency of the intermediate signals is at a value close to 344 MHz.
The solution proposed in JP Patent No 05-244032 also greatly complicates the structure of this communication device in particular by using several bandpass filters. This device provides no solution for reducing noise linked to the image frequency. The frequency value of the first high frequency signals mixed with the received and filtered radio-frequency signals is chosen only so that the frequency of the intermediate signals at the output of the first mixer is always the same. Consequently no particular means are provided simply for reducing the noise linked to the image frequency. Moreover, a communication device of this type uses a great deal of electrical energy, and thus cannot be fitted to a small portable instrument powered by a battery or an electrical energy accumulator.
It is thus a main object of the invention to provide a low power radio-frequency signal reception and/or transmission device of simple design for easily reducing the noise mainly linked to the image frequency of the converted signals in order to overcome the aforecited drawbacks of the prior art. Moreover, as many as possible of the elements of the simply structured device must be able to be integrated in a single integrated circuit for the sake of saving space.